Question: 7 folders cost $7.70. Which equation would help determine the cost of 4 folders?
Answer: There are several equations that could help determine the cost, each with a slightly different approach. We know the cost of 7 folders. We want to know the cost of 4 folders. We can write the numbers of folders as a proportion: $\dfrac{7}{4}$ We know 7 folders costs $7.70. We can let $x$ represent the unknown cost of 4 folders. The proportion of these costs can be expressed as: $\dfrac{\$7.70}{x}$ The cost changes along with the number of folders purchased, and so the two proportions are equivalent. $\dfrac{7}{4} = \dfrac{\$7.70}{x}$